Large-scale optical trapping using a gradient-thickness protected microbottle resonator.

Despite its huge potential, such as in biomedical research for bioparticle sorting and sensing, near-field optical trapping suffers from limited trapping efficiency due to the weak evanescent field accompanied by shallow penetration depth (~100 nm). Moreover, such optical trapping approaches are susceptible to perturbations from trapped particles, making them less robust and impractical. Here, we demonstrate, for the first time, a thin-walled hollow microbottle resonator with gradient-wall thickness to realize large-scale and robust optical trapping based on mode field strength antinodes, instead of the evanescent field. The microbottle resonator combined with off-equatorial fiber taper coupling collaboratively enables the excitation of axial high-order Whispering Gallery Modes (WGMs). In addition, the unique feature of the gradient-wall thickness design mitigates the adverse impact of the perturbation from trapped particles on mode field distributions, making the gradient-thickness protected (GTP) microbottle resonator more robust and stable. This enables large-scale optical trapping over an axial span exceeding 195 μm, with a threshold power of 0.198 mW for 500-nm-radius polystyrene particles. The GTP WGM microbottle resonator also achieves tunable localized optical trapping. This work demonstrates a scalable optical manipulation framework for applications in single-particle analysis, bioparticle manipulation, and label-free sensing.